Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells, which are a type of photosensitive optoelectronic device, are specifically used to generate electrical power. PV devices, which may generate electrical power from light sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
More recently, optoelectronic devices that make use of organic materials have become increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic optoelectronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Organic materials have manufacturing process advantages relative to inorganic thin-film technologies enabling the ability to apply organic material layers to plastic substrates. Examples of organic optoelectronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells (OPVs), and organic photodetectors. As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic optoelectronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. For OLEDs, the organic materials therein may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entireties.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic optoelectronic device. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated herein by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may be comprised of a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent. Side emitting OLEDs may also be fabricated, and one or both electrodes may be opaque or reflective in such devices.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” or “deposited over” a second layer, the first layer is disposed or deposited further away from the substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
Solar cells can be characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater under solar illumination. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. Due to the manufacturing and stability issues associated with crystalline or amorphous silicon devices, the cost of the electrical power produced per unit area has been high. The use of current inorganic solar cells is currently limited by their high manufacturing costs relative to the electrical power produced per unit area, as well as their high installation costs resulting from the cumbersome nature of the cell frames, the remainder of system equipment, and the panels themselves. Thus, a need exists for a more economical method of manufacturing PV cells having an acceptable quantum yield and therefore, power conversion efficiency.
More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs. In organic materials, light absorption leads to the formation of excitons, or bound electron-hole pairs, rather than the free electron-hole pairs produced in inorganic semiconductors. Efficient dissociation of excitons in organic materials occurs under strong electric fields, or at a donor-acceptor (DA) heterointerface where the differences in the electron affinities and the ionization potentials between the contacting organic materials are sufficiently large to overcome the exciton binding energy. The later mechanism has been employed to form an organic DA planar heterojunction (HJ) photovoltaic cell with a power conversion efficiency ηP˜1%, which was limited by exciton diffusion lengths (LD≦100 Angstroms) being much shorter than the optical absorption length (LA˜1000 Angstroms). Employing C60 (see U.S. Pat. No. 6,580,027) as the acceptor material, which has LD˜400 Angstroms, a power conversion efficiency ηP˜3.6% has been demonstrated. Because excitons generated within an exciton diffusion length of the DA heterojunction are subject to the electric field of the junction, they generally have a higher probability of dissociating efficiently. Conversely, excitons generated more than an exciton diffusion length from the DA heterojunction generally have a smaller probability of dissociating efficiently and contributing to device current. Bulk heterojunctions seek to create a highly interfolded or interpercolating network of the donor and acceptor materials such that an exciton generated by the absorption of incident radiation is likely close to a heterojunction and is likely to disassociate efficiently contributing to device current.